The invention relates to control arrangements. More particularly, this invention concerns a control apparatus for a hydraulic power consumer or user, particularly in mobile agricultural machines.
Conventionally, (see for example German Offenlegungsschrift No. 22 32 857) such an arrangement includes a reversing or switching valve actuated by a magnetic valve and operative to supply pressure medium to a user. The reversing valve is provided with a relief valve. In order to reverse (e.g. lower) the movement of the user there is provided a conduit having a second valve which is connected with a preliminary control valve or a servo-valve for operating this second valve.
A shortcoming of such a construction resides in providing two different valves which are necessary to alternately supply and withdraw the pressure medium to and from the user, respectively. That results in relatively high expenses and makes the possibility of leakage in such a system very likely. Besides, it raises the danger of a situation where the malfunctions can occur. Also, when the load pressure is rather small it is possible to lower the user only very slowly. Furthermore, the danger exists that one valve is not yet closed before the other already opens, with the resultant undesirable consequences. Also, it is possible in such constructions, when the load pressure is rather small in the user and the neutral pressure is high, that an undesired and unpredictable lifting of the user may take place. Utilization of two different servo-valves raises the expenses of such an arrangement. Besides, it has been recognized that these arrangements are also not satisfactory with respect to the strict requirements made as to the precision of the controlling operation.